Fix debug assertion for diagnostic sort order (#4319)

The debug assertion that document diagnostics are sorted incorrectly
panics for cases like `[161..164, 162..162]`. The merging behavior
in the following lines that relies on the assertion only needs the
input ranges to be sorted by `range.start`, so this change simplifies
the assertion to only catch violations of that assumption.

0 files changed, 0 insertions, 0 deletions